Syllabus Coverage: NFC4.6
The following table provides a consolidated mapping of the principal radiological hazards from irradiated fuel to their specific control measures across all stages of the back end of the fuel cycle. This summary draws together material from Chapters 4 and 5 to support syllabus outcome NFC4.6.
| Hazard Category | Storage (Pool/Dry) | Transport | Reprocessing | Disposal |
|---|---|---|---|---|
| External radiation (gamma/neutron) | Water shielding (≥2.5 m); concrete/steel for dry stores; gamma monitors with 2/3 voting logic | Flask walls (360 mm steel); IAEA Type B package limits (2 mSv/h surface) | Remote operation behind heavy concrete/steel shielding; hot cells | Multi-barrier system; deep geological emplacement (200–1000 m) |
| Criticality | Spacing racks; neutron poisons (boron) in water/racks; seismic-resistant design; geometry control | Flask geometry and neutron poisons; water ingress analysis; criticality safety assessment | Favourable geometry vessels; batch control; gadolinium neutron poison; monitoring and accountancy | Waste form limits fissile concentration; canister spacing; backfill material |
| Decay heat | Continuous water circulation (pool); passive air cooling with fins (dry) | Flask design permits passive heat dissipation; thermal analysis per IAEA requirements | Cooled storage tanks for HA liquor; vitrification process manages heat load | Interim cooling period (≥50 years) before disposal; repository ventilation during operational phase |
| Gaseous/volatile fission product release | Air extraction above pond; continuous air monitoring; pond water beta counter for early warning | Containment integrity of flask (leak-tested); double-lidded design | Off-gas scrubbing (>99.99% removal); HEPA filtration; iodine traps; condensation | Waste form immobilises volatiles; canister provides containment; buffer limits migration |
| Contamination (surface/airborne) | Water purification systems; ventilation with filtration; controlled access | Surface contamination limits on packages; swipe testing before dispatch | Glove boxes; negative pressure cascades; decontamination procedures; HEPA ventilation | Waste conditioning (vitrification/cementation) immobilises contamination |
| Long-term containment failure | Structural integrity monitoring; corrosion control of fuel cladding | N/A (short-duration transport) | N/A (material processed and conditioned) | Multi-barrier approach: waste form + canister + buffer + host rock; each barrier independent |
Key Point: The overriding principle across all stages is defence in depth — multiple independent barriers and controls ensure that no single failure can lead to a significant radiological consequence. The controls are proportionate to the hazard: the highest-activity stages (reprocessing, early storage) have the most extensive engineered controls, while later stages (disposal) rely increasingly on passive safety provided by the waste form and geological barriers.